Changes in the chemical composition of soil organic matter over time in the presence and absence of living roots: a pyrolysis GC/MS study

Aim To determine changes in the organic matter chemical signature of soils incubated in the absence of fresh organic matter input, and how these were affected by the reestablishment of vegetation. Methods An Alfisol and an Andisol were incubated in 1.50 dm3 PVC pots for 295 days. Thereafter two 0.65 dm3 undisturbed subsamples from each pot were taken. In one subsample, Medicago sativa L. was seeded; in the other, the incubation was continued without plants for an additional period of 215 days. Soils sampled at times 0, 295 d and 510 day were characterised using pyrolysis-GC/MS. Results and Conclusions During the first 295 days (in which plants were absent) the most evident changes detected were the degradation of the most labile fraction as shown by the decrease of pyrolysis products of plant-derived polysaccharides, intact lignin and long-chain aliphatic compounds, along with the residual accumulation of guaiacol, mid- to short-chain aliphatic compounds, and the aromatic fraction. On day 510 and in the absence of plants, fingerprints of lignin and plant-derived polysaccharides largely decreased whilst microbial-derived polysaccharides showed an accumulating trend. Moreover the relative contribution of n-methyl ketones increased whereas that of long-chain aliphatic compounds, specifically n-alkanes, decreased. The relative contribution of plant-derived compounds was larger in the Alfisol at T0 and decreased more intensely than the Andisol along incubation. The Andisol had a considerable fraction of microbial-derived compounds (e.g., acetamide and diketopiperazine compounds). Plant inclusion (during the last 215 days of incubation) increased (i) the presence of compounds associated with fresh plant detritus (e.g., plant-derived polysaccharides and lignin) and (ii) alkylated benzenes (likely root-derived). An enhanced microbial activity due to input of plant detritus could be inferred from the increased content of chitin-derived compounds, this being especially evident in the Andisol.